1. Field of the Invention
The present invention relates to hydrocarbon processing in general and, more particularly, to catalytic cracking of hydrocarbonaceous feedstocks. Specifically, the present invention relates to a process for catalytically cracking a hydrocarbonaceous feedstock with a dual component catalyst composition comprising an essentially alumina-free crystalline chromia silicate and a large pore size crystalline aluminosilicate cracking component. In a preferred embodiment, the essentially alumina-free crystalline chromia silicate is the crystalline chromia silicate more fully described in my copending application, Ser. No. 160,618, filed June 25, 1980.
2. Brief Description of the Prior Art
Catalytic cracking systems typically employ a fluidized bed or a moving bed of a finely divided particulate catalyst. This cracking catalyst is subjected to continuous cycling between a cracking reaction and a catalyst regeneration system. In a fluidized catalytic cracking (FCC) system, a stream of a hydrocarbonaceous feedstock is generally contacted with fluidized catalyst particles in a reaction zone, usually at a temperature of from about 425.degree.-600.degree. C., or higher. The cracking of the hydrocarbons in the feed generally results in deposition of carbonaceous coke on the catalyst particles and, of course, in the production of lower-molecular-weight hydrocarbons. The hydrocarbons are separated from the catalyst which is stripped of volatiles and passed to the catalyst regenerator where it is contacted with an oxygen-containing gas to burn off the coke. The heat evolved during this coke burnoff heats the catalyst particles and supplies the sensible heat for the cracking reaction. The thus regenerated catalyst is returned to the reaction zone for contacting additional feedstock.
Zeolitic materials, both natural and synthetic, are known to have catalytic activity in various types of hydrocarbon conversion reactions including catalytic cracking. Molecular sieve crystalline zeolites are aluminosilicates comprised of a rigid 3-dimensional framework of SiO.sub.4 and AlO.sub.4 tetrahedra joined by common oxygen atoms. The inclusion of aluminum atoms in the framework produces a deficiency in electrical charge which must be locally neutralized by the presence of additional positive ions within the structural framework. In natural zeolites, and many of the synthetic zeolites, these ions are normally alkali metal or alkaline earth metal cations which are quite mobile and readily exchanged. The cations occupy channels and interconnected voids provided by the framework geometry. U.S. Pat. No. 3,758,403, to Rosinski et at., the disclosure of which is incorporated by reference herein, describes the ZSM-5-type zeolitic aluminosilicate catalysts and their preparation in detail.
The present invention is related in part to crystalline silicate catalysts which are essentially alumina free. In addition to the chromia silicates which are the subject of my copending U.S. patent application Ser. No. 160,618, filed June 25, 1980, other essentially alumina-free crystalline silicates have been prepared and reported in the literature. U.S. Pat. No. 4,073,865 to Flanigen et al., incorporated by reference herein, discloses various crystalline silica polymorphs and methods for their preparation. U.S. Pat. No. 4,061,724 to Grose et al., incorporated by reference herein, discloses a crystalline silica polymorph called "silicalite" and a method for its preparation. U.S. Pat. No. Re. 29,948 to Dwyer et al., incorporated by reference herein, discloses a crystalline silicate essentially free of Group IIIA metals, a method for its preparation and processes employing the same.
Flanigen et al, Nature, 271, 512-516 (Feb. 9, 1978) discuss the physical and adsorption characteristics of silicalite. Bibby et al., Nature, 280, 664-665 (Aug. 23, 1979) report the preparation of a crystalline silicate denominated "silicalite-2". Finally, Anderson et al., J. Catalysis, 58 114-130 (1979) discuss catalytic reactions and sorption measurements carried out on ZSM-5 and silicalite.
In addition to an essentially alumina-free catalyst component, the catalyst of the present invention also employs a large pore size crystalline aluminosilicate cracking component. Such components are well known. Briefly, a crystalline aluminosilicate cracking component has a uniform pore dimension and a pore size from about 7 to 15 Angstroms. These large pore size cracking components admit both normal and iso-aliphatics and have the capability of acting with respect to substantially all the components of a gas oil feed. Zeolites, including the synthetic faujasites, known as zeolite X and zeolite Y, are particularly desirable. Of course, other large pore size zeolites can be employed.
Combination catalysts have been employed to treat hydrocarbon feedstocks. U.S. Pat. No. 3,686,121 to Kimberlin, Jr. et al discloses a hydrocarbon conversion catalyst comprising at least two crystalline aluminosilicate zeolites having essentially the same crystal structure but having different silica-alumina molar ratios, e.g., mixtures of zeolites X and Y. U.S. Pat. No. 3,748,251 to Demmel et al., discloses a catalyst composition comprising two cracking components, one of which is a ZSM-5-type zeolite. Finally, U.S. Pat. No. 3,847,793 to Schwartz et al., discloses a two-stage combination cracking operation which relies upon a combination of catalyst functions and a dual cracking component catalyst comprising a ZSM-5-type of crystalline aluminosilicate composition.